1. Field of the Invention
The present invention relates to an apparatus and process for the separation of normal butane and isobutane by fractionation. More particularly, the invention relates to the utilization of a heat pump and associated equipment to supply at least a portion of the energy necessary for the fractionation.
2. Prior Art
The use of heat pumps in the treatment of various refinery streams is known, for example heat pumps have been used in specific applications to separate ethane/ethylene and propane/propylene mixtures. Heat pumps have been suggested for open and closed heat pump cycles for low temperature separations of tertiary mixtures. For example, U.S. Pat. No. 3,509,728 discloses the use of an open heat pump cycle wherein components which are separated in the system are employed as refrigerants.
The present invention is directed to the conservation of energy by the use of an open heat pump cycle to supply a portion of the heat to a n-butane/isobutane splitter using n-butane from the splitter as the compression fluid in the heat pump. However, both n-butane and isobutane on compression result in a mixed liquid-vapor phase isentropic mixture. This is unlike ethane and propane or for that matter most hydrocarbons.
The liquid can be very damaging to the compressor and the present invention has overcome this problem and produces an energy savings in the energy otherwise required to operate the splitter.
It is a principal feature of the present invention that existing n-butane and isobutane splitters can be readily adapted to employ the present heat pump cycle either in place of or as a supplement to existing systems of supplying heat to the splitter.
An advantage of the present heat pump cycle is the use of n-butane as the heat pump fluid or medium, since it requires a lower energy input to obtain the same total heat transfer compared to isobutane.
It is an advantage of the present heat pump cycle that it is an open cycle employing therein a material (n-butane) from the system in which it is employed. It is an advantage of the present open heat pump cycle that the energy saving is transmitted to the splitter directly by return of heat pump fluid to the splitter.
It is a feature of the present invention that means have been provided to eliminate the formation of a liquid phase in the butane on compression in the heat pump.
It is a further feature of the present invention that heat is also recovered from the isobutane fraction from the splitter.
It is a particular advantage of the present invention that a substantial energy savings in operation of a C.sub.4 splitter is realized. In particular, the present invention provides a means to operate at less than 20% of the outside or incremental energy used to operate the splitter in the conventional manner. These and other advantages and features will become apparent from the following descriptions.